Perovskite solar cells (PSCs) are a promising candidate for next-generation photovoltaics, demonstrating remarkable advances in performance during the last decade, with record power conversion efficiencies (PCEs) exceeding 26%. Vacuum deposition techniques are widely used for fabrication of thin-films and have several advantages compared to solution-based fabrication methods. These include conformal deposition of high-quality layers, low material consumption and the ease of scalability to larger areas. However, PCEs of thermally evaporated PSCs have been lacking behind those of solution-processed PSCs for years. While originally most research regarding thermally evaporated PSCs was dedicated to co-evaporation processes, reaching maximum PCEs of 20.6%,^[1] recent record n-i-p PCEs > 21% were achieved via sequential (two-step) layer deposition approaches.^[2,3] Here, the individual perovskite precursors are deposited in two steps and converted to the final perovskite film during a subsequent annealing step.

In this work, we present a sequential evaporation process to fabricate all-vacuum-processed methylammonium-free PSCs in the p-i-n architecture. In the first process step, the inorganic layer is deposited onto the substrate, followed by the deposition of formamidinium iodide (FAI) in the second evaporation step. The conversion of these two layers into the final perovskite film is performed during an optimized annealing step under ambient atmosphere. We will present phase-pure formamidinium lead iodide (FAPI) PSCs using an all-vacuum-processed layer stack with PCEs above 16%, among the highest reported for evaporated FAPI PSCs in the p-i-n architecture.

Interestingly, we observe a significant difference in X-ray diffraction measurements of the final perovskite thin film when changing the underlying hole transport layer (HTL). Further experiments demonstrate variations in microstructure and morphology of the inorganic layer on various HTLs, which impact the interaction with FAI and the conversion into a perovskite film. Proving this substrate-dependent film growth and understanding how to manipulate the microstructure/morphology of the inorganic layer by adjusting process parameters during the evaporation marks a huge step forward in understanding the sequential evaporation process. We will discuss these findings at the conference.

Furthermore, addition of further inorganic precursor materials (lead bromide, caesium iodide, caesium bromide) in the first evaporation step allows adjusting the bandgap, facilitating fabrication of efficient wide-bandgap PSCs and perovskite-based tandem solar cells. Our work paves the way for efficient all-evaporated PSCs and their application to monolithic tandem solar cells with an up-scalable and industrially relevant deposition technique.